zstd: Handle dicts by pointer, always (#743)

dict structures were mostly handled by pointer, except in one place.
Since this doesn't make them immutable (they contain pointers to other
structs), just copy pointers always to save some memory.

zstd/decoder.go

@@ -40,8 +40,7 @@ type Decoder struct {
 	frame *frameDec
 
 	// Custom dictionaries.
-	// Always uses copies.
-	dicts map[uint32]dict
+	dicts map[uint32]*dict
 
 	// streamWg is the waitgroup for all streams
 	streamWg sync.WaitGroup
@@ -103,7 +102,7 @@ func NewReader(r io.Reader, opts ...DOption) (*Decoder, error) {
 	}
 
 	// Transfer option dicts.
-	d.dicts = make(map[uint32]dict, len(d.o.dicts))
+	d.dicts = make(map[uint32]*dict, len(d.o.dicts))
 	for _, dc := range d.o.dicts {
 		d.dicts[dc.id] = dc
 	}
@@ -942,7 +941,7 @@ func (d *Decoder) setDict(frame *frameDec) (err error) {
 		if debugDecoder {
 			println("setting dict", frame.DictionaryID)
 		}
-		frame.history.setDict(&dict)
+		frame.history.setDict(dict)
 	} else if frame.DictionaryID != 0 {
 		// A zero or missing dictionary id is ambiguous:
 		// either dictionary zero, or no dictionary. In particular,

zstd/decoder_options.go

@@ -20,7 +20,7 @@ type decoderOptions struct {
 	concurrent      int
 	maxDecodedSize  uint64
 	maxWindowSize   uint64
-	dicts           []dict
+	dicts           []*dict
 	ignoreChecksum  bool
 	limitToCap      bool
 	decodeBufsBelow int
@@ -101,7 +101,7 @@ func WithDecoderDicts(dicts ...[]byte) DOption {
 			if err != nil {
 				return err
 			}
-			o.dicts = append(o.dicts, *d)
+			o.dicts = append(o.dicts, d)
 		}
 		return nil
 	}
@@ -114,7 +114,7 @@ func WithDecoderDictRaw(id uint32, content []byte) DOption {
 		if bits.UintSize > 32 && uint(len(content)) > dictMaxLength {
 			return fmt.Errorf("dictionary of size %d > 2GiB too large", len(content))
 		}
-		o.dicts = append(o.dicts, dict{id: id, content: content, offsets: [3]int{1, 4, 8}})
+		o.dicts = append(o.dicts, &dict{id: id, content: content, offsets: [3]int{1, 4, 8}})
 		return nil
 	}
 }